When cement hydrates, water binds with the compounds
contained in cement on a molecular level. The main product of hydration is CSH,
the molecules of which become interconnected; giving concrete its cohesion.
In the early stages of hydration ettringite is formed,
which provides concrete with its early setting capacity. Due to the abundance
of sulphate ions present this tri-sulphate is easily made, however is likely
then broken down to a monosulphate compound, which has less effect on the strength
of the concrete.
With the inclusion of cement replacing materials in the
cement the hydration process could be extended. Pozzolans such as fly ash and
slag, despite their concrete improving properties, can hydrate slower than
Portland cement; which must be considered before the concretes placement.
Hydration is an
exothermic reaction, meaning the chemical reaction releases heat energy. In
large pours, this can cause cracking and noticeable shrinkage.